The proposed research is related to Research Objective #11, Sensory and Motor Processing. Parkinson's disease (PD) is a progressive age-related neurodegenerative disorder whose pathological hallmark is the degeneration of dopaminergic neurons in the substantia nigra pars compacta and depletion of striatal dopamine levels. As a result of this degeneration, neuronal activity of the primary output structure of the basal ganglia, the internal segment of the globus pallisus (GPi), is increased in an aberrant fashion and development of the cardinal motor features of PD, including akinesia, bradykinesia, tremor, and/or rigidity, occurs. Although its mechanisms remain unclear, deep brain stimulation (DBS) of the GPi or subthalamic nucleus (STN) provides a promising surgical approach in the alleviation of parkinsonian motor signs. However, the relative novelty of DBS has precluded the systematic investigation of the effects of DBS in GPi or STN on motor performance in advanced PD patients. The primary goal of this project is to determine effects of DBS in STN or GPi on force control in advanced PD patients. It is hypothesized that basal ganglia dysfunction, as exhibited in advanced PD, adversely affects PD patients ability to precisely control forces produced by the digits. Furthermore, it is hypothesized that DBS of GPi or STN improves force control capabilities of PD patients by "normalizing" neuronal output from GPi. Patients will use a precision grip (thumb and index finger only) to perform a force-tracking task in which they will be required to produce and maintain a wide range of target forces (5-50% of maximum). Data will be collected from 10 STN and 10 GPi patients prior to stimulator implantation. Approximately six months after stimulator impantation, patients will perform the same force-track task. Force (kinetic) and clinical measures will be used to determine the changes in force control and overall motor function. Data collected in this study will expand our understanding of the role of the basal ganglia in force control. Upon completion of this project, recommendations will be made regarding the optimal stimulation site for improving the control of digit forces of in advanced PD patients. The results of these studies may improve the treatment of advanced PD.